1. Field of the Invention
The present invention relates to a liquid crystal display and, more particularly, to a liquid crystal display using a polymer dispersed liquid crystal (PDLC).
2. Description of the Related Art
The polymer-dispersed liquid crystal has a structure in which the liquid crystal is phase-separated in a polymer formed into a three-dimensional network (polymer network), and creates a light scattering state and a light transmission state by a state in which the liquid crystal molecules are arranged at random along the side walls of the polymer network and a state in which an electric field is applied to align the liquid crystal molecules in the direction of the electric field, respectively (for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-19583). The degree of scattering is decided by the voltage applied to the liquid crystal. The voltage applied to the liquid crystal is decided by voltage division between the polymer material and the liquid crystal.
FIG. 20 is a sectional view showing an example of a conventional liquid crystal display. A transparent display electrode 102 is formed on one glass substrate 101. A transparent common electrode 104 is formed on the other glass substrate 103. The region where the display electrode 102 is formed is a region capable of displaying a pattern. The above-described two glass substrates 101 and 103 sandwich a polymer-dispersed liquid crystal layer 105. A seal member 106 bonds the two glass substrates 101 and 103 so as to seal the polymer-dispersed liquid crystal layer 105. The liquid crystal display shown in FIG. 20 displays a pattern by applying a voltage between the display electrode 102 and the common electrode 104. For example, when a voltage of 5 V is applied between the display electrode 102 and the common electrode 104, a transparent state is obtained. When the voltage between the display electrode 102 and the common electrode 104 is reduced to zero, a scattering state is obtained.
FIG. 21 is a graph showing the relationship between the voltage and the transmittance replacing the degree of scattering at a plurality of temperatures (for example, −10, 25, and 50° C.) As can be understood from FIG. 21, the transmittance at the same voltage largely changes depending on the temperature. The reason for this is as follows. When the temperature is high, the dielectric constant of the liquid crystal is small. Conversely, when the temperature is low, the dielectric constant is large. However, the dielectric constant of the polymer material changes little if the temperature difference is several tens of degrees centigrade. Since the effective voltage actually applied to the liquid crystal is divided by the polymer material, the effective voltage is high at a high temperature and low at a low temperature. For this reason, the characteristic of the liquid crystal display using the polymer-dispersed liquid crystal largely changes depending on the temperature, as shown in FIG. 21.
When performing binary display of scattering (0 V) and transmission (5 V), a change in the temperature does not influence the display. However, when expressing halftone by the voltage, the transmittance largely changes depending on the temperature. For example, when the voltage is 2.2 V, the transmittances are 50% at 25° C., 86% at 50° C., and 5% at −10° C. The halftone cannot be expressed at 50 and −10° C., and display equivalent to binary display is only performed.
In addition, Jpn. PCT National Publication No. 2003-533751 discloses a technique of displaying halftone by dithering.